Automatic container handling is typically accomplished by means of a crane having a generally rectangularly shaped movable trolley 10 located on a repositionable frame 15 as shown in FIG. 1. A generally rectangularly shaped spreader 20 is used both to move containers 25 onto a stack located on the ground beneath the frame 15 and to pick up containers 25 from such a stack. Spreader 20 is typically connected to trolley 10 by means of cables for raising and lowering the spreader. In order to avoid creating an unstable, leaning stack while loading and to efficiently pick up containers and since spreader 20 moves independently of trolley 10, it is critical that the location of spreader 20 in relationship to trolley 10 be determined as accurately as possible at specific points in time and that the location of the stack also be determined as accurately as possible. Such position data is typically obtained by means of a vision scanner system to guide and control movements of trolley 10 and spreader 20 during pick-up and drop-off of containers in a stack. An example of such a system is the Maxview® Crane Scanner System developed by TM GE. Maxview® is a registered trademark belonging to TM GE Automation Systems LLC. Maxview® uses a combination of sensors and TM GE proprietary software to measure the relative position of the spreader and container below the crane, as is generally illustrated in FIG. 1. In the typical system used in automatic container handling, a crane computer controller provisionally positions frame 15 and trolley 10 at the expected position of container 25. An exemplary arrangement for such a control system is presented in FIG. 2. The expected position is retrieved from a database maintained by the crane computer controller. Once frame 15 and trolley 10 arrive at the expected position, some means must be used to more precisely, finally position spreader 20 over container 25. A variety of systems have been used to accomplish this goal.
LIDAR (Light Detection and Ranging) devices, also referred to as “laser sensors” or “laser scanners”, are one possibility. Sensors 30 (not visible in FIG. 1) are typically affixed at a known position to trolley 10 and provide a distance and angle measurement of a series of points from the reference point of the sensor. Such sensors measure distance via the “time of flight” method. Each LIDAR sensor 30 emits a pulse of laser light, and then precisely measures the time until the pulse, or some part of it, is reflected back. This time is then multiplied by the speed of light to obtain a distance measurement. So-called laser rangefinders use this technique to make one-dimensional distance measurements.
FIG. 2 illustrates the operation of this type of system for a one-dimensional distance measurement. At time t=0, the LIDAR sensor 30 begins to transmit (Tx) a first laser pulse 40 towards target 45 as shown in the left-hand portion of FIG. 2. Target 45 is located an unknown vertical distance d away from sensor 30 as shown in the right-hand portion of FIG. 2. Laser pulse 40 reaches target 45 in one-half of the time (T) necessary for the roundtrip of pulse 40 back to LIDAR sensor 30 as shown in the middle portion of FIG. 2. When pulse 40 is received (Rx) back at LIDAR sensor 30, the total time T for its transit time is known, as shown in the right-hand portion of FIG. 2, and is recorded. The distance d can then be calculated according to the formula:d=½T*Speed of Light
The laser scanner can also be used to take the measurement concept one step further—into two-dimensions. A rotating mirror 35 (not visible in FIG. 1) which is physically integrated into sensor 30 is used to direct the laser pulses over a range of angles. This process provides individual distance measurements at the corresponding mirror angles to different profile points. These points form a two-dimensional profile, or cross-section, of the area being scanned. In FIG. 3, rotating mirror 35 is used to rapidly direct a plurality of laser pulses originating in LIDAR sensor 30 over a range of angles towards target 45. As a result, data from a multiplicity of measuring points 50 are obtained enabling distance measurements to be obtained corresponding to the mirror angles and a two-dimensional profile, or cross-section, of the scanned area to be developed. The TM GE proprietary Crane Scanner System software receives the discrete scan point measurements provided by the laser scanners, detects the edges of key objects within the laser scans, and reports measurements of these edge positions in various coordinate systems to the Maxspeed® crane control system. Maxspeed® is a registered trademark belonging to TM GE Automation Systems LLC. The Maxspeed® system uses these measurement positions to guide the spreader to the target container for automatic container pick-ups and landings (stacking).
As shown in FIG. 4, the known techniques of edge detection and measurement are limited in accuracy by the scan point resolution of the laser scanner. Spot-to-spot resolution ss (the difference between the centers of two adjacent spots) and spot diameter sd of commercially available, robust and economical laser scanner units suitable for use in crane applications cause large measurement uncertainties relative to the measurement accuracies required for automatic container handling. Actual edge positions x measured with such laser units differ substantially from measured edge positions y resulting in an undesirable degree of measurement uncertainty z. Furthermore, spot diameter, as shown on the y axis of the graph depicted in FIG. 5, becomes larger as the range, shown on the x axis of the graph depicted in FIG. 5, increases. This increase occurs since the laser beam being used in this application is intentionally given angular beam divergence at least as great as the angular resolution desired by appropriately setting the laser cavity geometry and employing matching optics within the path of light in the LIDAR itself. As a result, assurance is obtained that any object within the total scan angle will be hit by at least some part of the laser pulse. In addition, as the range increases, both the spot diameter and the distance between spots in millimeters increases, as shown in FIG. 6, thereby further reducing scanner accuracy using known methods. Further support for this conclusion is provided by “Technical Description—LMS 200/211/220/221/291 Laser Measurement Systems,” SICK AG, 8 008 970/WU.FD/06-2000, p. 6, FIG. 3-2.
What is needed, therefore, is a system and method for using pulse emitters such as laser scanners on existing cranes which improves the accuracy of target edge detection to overcome the problems discussed above.